RE: Performance data of Linux native vs. Xen Dom0 and Xen DomU. Re: [Xen-devel] Direct I/O to domU seeing a 30% performance hit

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> I'm also doing some performance analysis about Linux native, dom0 and
domU
> (para-virtualized). Here are some brief comparison for 256K sequential
> read/write. The testing is done using for JBOD based on 8 Maxtor SAS
Atlas
> 2
> 15K drives with LSI SAS HBA.
> 
> 256K Sequential Read
> Linux Native: 559.6MB/s
> Xen Domain0: 423.3MB/s
> Xen DomainU: 555.9MB/s

This doesn't make a lot of sense. Only thing I can think of is that
there must be some extra prefetching going on in the domU case. It still
doesn't explain why the dom0 result is so much worse than native. 

It might be worth repeating with both native and dom0 boot with
maxcpus=1.

Are you using near-identical kernels in both cases? Same drivers, same
part of the disk for the tests, etc?

How are you doing the measurement? A timed 'dd'?

Ian

 
> 256K Sequential Write
> Linux Native: 668.9MB/s
> Xen Domain0: 708.7MB/s
> Xen DomainU: 373.5MB/s
> 
> Just two questions:
> 
> It seems para-virtualized DomU outperform Dom0 in terms of sequential
read
> and is very to Linux native performance. However, DomU does show poor
(only
> 50%) sequential write performance compared with Linux native and Dom0.
> 
> Could you explain some reason behind this?
> 
> Thanks,
> 
> Liang
> 
> 
> ----- Original Message -----
> From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
> To: "John Byrne" <john.l.byrne@xxxxxx>
> Cc: "xen-devel" <xen-devel@xxxxxxxxxxxxxxxxxxx>; "Emmanuel Ackaouy"
> <ack@xxxxxxxxxxxxx>
> Sent: Tuesday, November 07, 2006 10:20 AM
> Subject: RE: [Xen-devel] Direct I/O to domU seeing a 30% performance
hit
> 
> 
> > Both dom0 and the domU are SLES 10, so I don't know why the "idle"
> > performance of the two should be different. The obvious asymmetry is
> the
> > disk. Since the disk isn't direct, any disk I/O by the domU would
> > certainly impact dom0, but I don't think there should be much, if
any.
> I
> > did run a dom0 test with the domU started, but idle and there was no
> > real change to dom0's numbers.
> >
> > What's the best way to gather information about what is going on
with
> > the domains without perturbing them? (Or, at least, perturbing
> everyone
> > equally.)
> >
> > As to the test, I am running netperf 2.4.1 on an outside machine to
> the
> > dom0 and the domU. (So the doms are running the netserver portion.)
I
> > was originally running it in the doms to the outside machine, but
when
> > the bad numbers showed up I moved it to the outside machine because
I
> > wondered if the bad numbers were due to something happening to the
> > system time in domU. The numbers is the "outside" test to domU look
> worse.
> 
> 
> It might be worth checking that there's no interrupt sharing
happening.
> While running the test against the domU, see how much CPU dom0 burns
in
> the same period using 'xm vcpu-list'.
> 
> To keep things simple, have dom0 and domU as uniprocessor guests.
> 
> Ian
> 
> 
> > Ian Pratt wrote:
> > >
> > >> There have been a couple of network receive throughput
> > >> performance regressions to domUs over time that were
> > >> subsequently fixed. I think one may have crept in to 3.0.3.
> > >
> > > The report was (I believe) with a NIC directly assigned to the
domU,
> so
> > > not using netfront/back at all.
> > >
> > > John: please can you give more details on your config.
> > >
> > > Ian
> > >
> > >> Are you seeing any dropped packets on the vif associated with
> > >> your domU in your dom0? If so, propagating changeset
> > >> 11861 from unstable may help:
> > >>
> > >> changeset:   11861:637eace6d5c6
> > >> user:        kfraser@xxxxxxxxxxxxxxxxxxxxx
> > >> date:        Mon Oct 23 11:20:37 2006 +0100
> > >> summary:     [NET] back: Fix packet queuing so that packets
> > >> are drained if the
> > >>
> > >>
> > >> In the past, we also had receive throughput issues to domUs
> > >> that were due to socket buffer size logic but those were
> > >> fixed a while ago.
> > >>
> > >> Can you send netstat -i output from dom0?
> > >>
> > >> Emmanuel.
> > >>
> > >>
> > >> On Mon, Nov 06, 2006 at 09:55:17PM -0800, John Byrne wrote:
> > >>> I was asked to test direct I/O to a PV domU. Since, I had a
system
> > >>> with two NICs, I gave one to a domU and one dom0. (Each is
> > >> running the
> > >>> same
> > >>> kernel: xen 3.0.3 x86_64.)
> > >>>
> > >>> I'm running netperf from an outside system to the domU and
> > >> dom0 and I
> > >>> am seeing 30% less throughput for the domU vs dom0.
> > >>>
> > >>> Is this to be expected? If so, why? If not, does anyone
> > >> have a guess
> > >>> as to what I might be doing wrong or what the issue might be?
> > >>>
> > >>> Thanks,
> > >>>
> > >>> John Byrne
> > >> _______________________________________________
> > >> Xen-devel mailing list
> > >> Xen-devel@xxxxxxxxxxxxxxxxxxx
> > >> http://lists.xensource.com/xen-devel
> > >>
> > >
> > > _______________________________________________
> > > Xen-devel mailing list
> > > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > > http://lists.xensource.com/xen-devel
> > >
> 
> 
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